Known electromagnets for high field intensities have a fixed soft iron frame, in which two co-axial poles with current coils are mounted rigidly. Their air gap is not adjustable.
Electromagnets are also known, which have a lower pole with a field coil connected rigidly to a fixed soft iron frame, and where the upper pole moves through an opening in the upper crossbar of the frame, thus enabling different air gaps to be adjusted. The field coil around the upper pole is rigidly connected to the crossbar of the frame. With these electromagnets, the maximum field strength is limited because the movement of the upper pole requires a field coil with cylindrical bore can only incompletely enclose the conic part of the upper pole.